HERTZIAN WAVES IN THE BASEMENT - CRYSTAL SETS TO SIDEBAND Frank W. Harris 2022, REV 16 - QSL.net
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1. Chapter 4, Harris
CRYSTAL SETS TO SIDEBAND
© Frank W. Harris 2022, REV 16
Chapter 4A
HERTZIAN WAVES IN THE BASEMENT
Crystal set radios and ancient spark gap transmitters from the 19th century may seem too
simple and too far removed from ham radio to be worth building. If you’re already familiar with
basic electronics and early radio history, you may not learn anything from this exercise. If you
can’t wait to build a real ham rig, please skip ahead to the next chapter. On the other hand, if you
have little electronics experience, there are worlds of lessons to be learned from old technology.
Or, if you’ve never built simple radios before, you might find it fun.
The nature of radio waves
Before we build transmitters and receivers, let’s review radio waves. When we
understand what radio waves are, the technology to generate and receive them becomes more
obvious. An electromagnetic wave is an oscillation in free space that radiates in all directions
away from its source at the speed of light. It is called electromagnetic because it is both electric
field energy and magnetic field energy. The wave oscillates or alternates back and forth between
these two forms of energy as it travels.
Propagating across vacuum
From our experiences with magnets and static electricity, it’s hard to visualize how a
magnetic or electric field can travel millions of miles across the vacuum of space. To us, these
fields always seemed tightly localized around the device that generated them. How can a
magnetic field exist isolated in a vacuum, perhaps even light years away from the nearest atom?
Suppose we could somehow magically generate a magnetic field or an electric field in space, miles
from the nearest object. Would the field just sit in space forever waiting for an object to pass by
and be influenced by the field?
Static electric and magnetic fields are forces which project out into the space surrounding
the battery or magnet that generated them. These forces are actual potential energy that exists
outside the generator that produced them. The energy is confined locally because of the battery
or magnet. Let’s suppose there is a refrigerator magnet floating in the void of space: Its
magnetic field will be at rest in the space surrounding the magnet, just as it does stuck to your
refrigerator. As always, the magnetic field will reach out its usual distance of a quarter inch or so.
However, if the magnet were to vanish suddenly, the energy in the field would lose its
“container” or “anchor” and be turned loose in the void.
The same scenario can be proposed for an electric field: If a flashlight battery were
floating in space, the electric force would extend perhaps an inch out into the space in a halo
around the two battery terminals. Again, if the battery suddenly vanished, the electric field energy
would lose its generator and be stranded in the void. Without its anchor, it would spill out in all
directions. I like the analogy of a glass of water sitting on a table. If the glass suddenly shatters,
the water will spill out in all directions.
2. Chapter 4, Harris
Oscillation occurs when two forms of energy change back and forth rhythmically
When magnetic and electric fields are turned loose in space, what becomes of them? As
James Maxwell first explained over a century ago, electric fields and magnetic fields are intimately
related. It turns out that a changing or moving electric field generates a changing magnetic
field and vice-versa. As the energy continues to “spill” out in all directions, the energy oscillates
back and forth between these two kinds of fields. This connection is not "obvious” or intuitive. If
it were, the Egyptians, Greeks or Chinese would have described and exploited it long ago.
Mechanical oscillators
Many physical devices in our world oscillate, so oscillation between electric and magnetic
fields should not be a surprise. An oscillation in nature can be described as energy spontaneously
transforming from one form of energy into another then back again. For example, as a clock
pendulum swings back and forth, the pendulum acquires the kinetic energy of motion as it swings
through the bottom of its arc. Then, when the pendulum swings back uphill, the energy contained
in the kinetic energy is returned to gravitational potential energy. When the pendulum reaches the
top of its swing, it momentarily comes to a complete halt, turns around and races back downhill.
When it is at the top, the energy is all “potential.” A boulder sitting on the edge of a cliff doesn’t
seem to have any energy until it is nudged off the cliff. The fellow standing at the bottom of cliff
can testify that the rock has plenty of energy when it slams into the foot of the cliff. (That
assumes, of course, that he survives.)
3. Chapter 4, Harris
To reiterate, an oscillating pendulum switches its energy back and forth between kinetic
energy and potential energy. Notice that the length of the pendulum establishes the oscillation
frequency of a pendulum, not the weight of the pendulum. This is because gravity is constant and
lightweight objects fall just as fast as heavy objects. If you ignore air resistance, the frequency of
the pendulum swing is determined solely by the length of the pendulum arm (and of course the
acceleration of gravity). This makes pendulums good for keeping a mechanical clock running
uniformly and accurately. Similarly, once the frequency of a radio wave is established, big signals
and small signals don’t change frequency as they race through space and become weaker.
In summary, free space (which is literally “nothing”), can support magnetic or electric field
energy, but only temporarily. To be maintained, a magnetic field needs to be generated by a
device. A magnetic field can be generated temporarily by a nearby collapsing electric field.
As the magnetic field collapses, it produces a temporary electric field in the adjacent space.
This seesaw produces a radio wave traveling outward across the void at the speed of light.
The speed of light
What's special about the speed of light? Good question! Einstein's laws of relativity teach
us that time and space are frozen like a 4 dimensional ice cube. Most people are aware that light
speed is the ultimate velocity. What is rarely understood is that light speed is really "the speed
of time itself." Relativity is almost never studied deeply, even by engineers. This is perhaps
because, until the invention of portable atomic clocks, the evidence proving that relativity was real
was fragmentary and not very convincing. For example, when the first GPS satellite was
launched, the software had "switches" that allowed ground engineers to turn off the adjustments
for Special and General Relativity. They did this because they thought relativity might be wrong.
Relativity was correct. The crazy theories turned out to be laws. Our universe is weird and
profound far beyond what 99.9% of humans have the privilege to notice. However, these subjects
belong in another book.
Getting back to 19th century radio: Transmitter antennas are designed to generate either a
rapidly changing electric field, or alternatively, a rapidly changing magnetic field. The antenna is
placed out in the open with free access to the sky. The changing electric or magnetic fields
around the antenna create the opposite kind of field and the result is a free-flying radio wave. The
4. Chapter 4, Harris
same antennas work well for receivers. As radio waves flash past the metal antenna elements,
electric currents are induced into the structure just as if it were temporarily a capacitor or the
secondary loop of a transformer.
The LC circuit, the fundamental electronic oscillator
The most fundamental component of all radio transmitters and receivers is the capacitor/
inductor parallel resonant circuit. This basic circuit consists of an inductor wired in parallel
with a capacitor. These are called LC circuits where L is the letter used when calculating
inductance and C of course stands for capacitance. If a high frequency sinewave voltage is
applied across the parallel LC circuit, there is a specific frequency at which the LC circuit
resonates and appears to be an open circuit. At all the other frequencies the LC appears as a load
or short circuit. The LC circuit attenuates or eliminates the sinewave at every frequency except
one. In this way one radio signal can be “tuned in” preferentially over another.
For example, the circuit above resonates at 14 MHz, the 20-meter hamband. The tiny
inductor is just 3.2 microHenries. The capacitor is only 40 picoFarads which means 40 millionths-
of-a-millionth of a Farad.
The LC circuit is a kind of electric oscillator. It is analogous to a swinging pendulum or a
5. Chapter 4, Harris
weight bouncing up and down on a mechanical spring. The LC oscillator goes through the same
energy cycle as radio waves. First the energy is stored in the magnetic field inside and around the
inductor. During the next half cycle, the energy is stored in an electric field between the plates of
the capacitor. The energy alternates back and forth between these components until the resistance
in the wires dissipates it.
A parallel resonant LC circuit with dual antennas forms a simple transmitter
Significant energy can also be leaked off into the space around the LC circuit as radio
waves. Therefore, once we are able to make an LC circuit oscillate, we are on our way to
generating radio waves. If we simply add wires to the ends of the parallel LC circuit, these
wires form an antenna to couple the electric field to the free space around it. In other words,
the simple circuit shown below is a crude radio transmitter.
A receiver of radio waves can be built in the same way. Imagine that an identical parallel
LC circuit with antennas is tuned to resonate at the same frequency. Now imagine that this
second circuit is floating in the void, perhaps miles from the transmitting circuit. When the radio
waves radiate past the receiving LC circuit, the electric field component in the radio wave will
produce a tiny surge of current in the wires that charge the capacitor. Alternatively, and
depending on the orientation of the coil with respect to the radio waves, the magnetic component
of the radio wave will induce a tiny voltage to appear across the coil. This is the same as if the
inductor were the secondary of a transformer. Once the radio wave has zoomed past, a tiny,
miniscule oscillation will remain in the receiving LC circuit, ringing back and forth between the
inductor and capacitor.
I bought a book at a local bookstore. Since I bought it to give as a birthday gift, I cut off
the price label. The label was a tough multiple-layered device of plastic, paper and foil. I peeled
it apart and discovered an LC "receiver" just like the diagram above. The capacitor was 2 small
squares of foil separated by a thin layer of plastic. The inductor was a spiral of foil printed on a
sheet of thin plastic. If I had attempted to leave the store without paying, a transmitter at the
door would have resonated with the receiver and set off an alarm.
******************************************************************************
6. Chapter 4, Harris
CRYSTAL SET RADIOS
A crystal set is an excellent first electronic project. They have few parts and are easy to
understand and build. Crystal sets were a common toy when I was a kid. The set below belonged
to my brother-in-law, Peter Hill, and probably dates from roughly 1940. I discovered it collecting
dust in the attic.
I connected the set to an antenna and ground. No matter how I adjusted it, it was as
silent as the attic it came from. A "crystal" usually consists of a tiny piece of lead-sulfide
crystalline ore imbedded in a small cylinder of solder. To my amazement, there was also a spare
crystal in its original little box. The spare was manufactured by the Clearco Crystal Company in
Idaho Springs, Colorado. The town is located in a canyon on Clear Creek. Idaho Springs is a
gold and silver mining town that had its boom time in the late 19th century. By the 1930s, Idaho
Springs was nearly a ghost town. The silver ore was a mixture of several metal-sulfides, primarily
lead-sulfide. The crystal is labeled "galena" which is the common name for lead-sulfide. It is
almost certainly the local ore and works as well as pure galena. Notice that they call their crystal
"the silver tone." The existence of this little company during the Great Depression is a tribute to
those creative fellows who found a way to make a living during a grim time for Colorado miners.
I installed the replacement crystal in the set but it remained silent. I happened to press the
earphone hard against my ear and heard a faint station. The tuning was surprisingly sharp, but it
wasn't nearly loud enough. For now I'll allow this historic radio to rest in peace.
As you might expect, my first electronic project was building a crystal set. Although we
kids played with crystal sets, we didn’t understand them. When they didn’t work, we had only the
haziest notion of how to fix them. I never saw a commercial toy crystal set that had explanations
even remotely as complete as what you are now reading. Starting with crystal set theory, we can
build on these concepts to build transmitters and more elaborate receivers.
7. Chapter 4, Harris
The combination of inductor and capacitor tune in the desired station. That is, the
inductor and capacitor oscillate at the frequency of the desired station. The crystal diode rectifies
the sinewaves oscillating across the LC circuit. "Rectification" clips off one polarity of the radio
frequency, alternating polarity sinewaves. This converts the high frequency sinewaves into low
frequency DC - direct current - which varies at sound wave frequencies. The earphone has too
much inductance to pass radio waves, but does admit low frequencies that can be turned into low
frequency voice and music. The magnet and coil in the headphones cause a steel diaphragm to
vibrate, generating the sound. Crystal sets have six basic parts. The antenna of course picks up
the signal from the air.
The antenna
For crystal sets it is easiest to build an “electric field” antenna. It is usually just a long
piece of wire strung out a window or up in a tree. A limitation of a single wire antenna like this is
that, when the radio wave generates a voltage on the wire, the current it might produce has no-
where to go. A simple wire antenna is like one terminal of battery. Yes, the battery has a voltage,
but without a connection to the other terminal of the LC circuit, the current has no circuit to flow
through. To provide a destination for the current we can add a second antenna. Alternatively we
can connect the crystal set to “ground.”
The ground
Electrical “ground” is a word that we learn as youngsters, but most people go through
their whole lives without ever understanding it. I suspect that the term arose during the early days
of telegraph communication in the 1840s. It turns out that wet earth is a fairly good conductor.
If you drive two metal stakes into the ground in your backyard and connect a battery to the two
stakes, current will flow from one stake to the other. For two stakes about 100 feet apart, the
ground has a resistance of about 100 ohms. If you experiment, you learn that most of the
electrical resistance to current flow occurs right around the stakes. Once the current gets
launched, the electrical resistance only increases slightly as you increase the distance. A metal
stake in a backyard in China and one in a backyard in the U.S.A. might only have 300 ohms
resistance between them. This was a boon for early telegraphers because it meant they only had
to string one wire between cities instead of two wires to complete the circuit loop. In practice,
using ground as an intercity “wire” is not as reliable as stringing a second wire, but it illustrates
the concept of ground.
8. Chapter 4, Harris
A good electrical connection with ground is an essential component of the crystal set. The
most accessible ground for a crystal set or a ham radio station is usually a copper water pipe or a
hot water heating pipe. At low Radio Frequencies, such as standard broadcast AM radio, ideal
electric antennas are very large. Needless to say, being able to use the entire Earth as half of the
antenna is often quite convenient.
Dipoles
Unlike low frequencies, for high frequencies, like VHF television or FM radio, the ideal
length for an electric antenna is just a meter or so. The higher the frequency, the less distance
there is between successive electric and magnetic propagating waves. That is, the wavelength
decreases with increasing frequency.
Although in theory you could use a stake driven into your lawn for the “ground” path
return for your TV, it is much simpler just to use a second short antenna oriented 180o away from
the “real” antenna. This dual antenna is called a dipole and is the most common basic antenna
design used in ham radio and TV antennas. Although it isn’t obvious, the arrays of thin aluminum
tubes on rooftop TV antennas are just refinements of the basic dipole antenna. In general, the
lower the frequency, the larger the dipole must be in order to work well. A typical ham radio
dipole is shown below.
9. Chapter 4, Harris
Transmission lines
In the ham radio dipole antenna above, the “arms” stick out in space in opposite directions
and snag the passing electrical field. In general, the higher the dipole is above the local terrain,
the better the dipole will receive signals. Unfortunately, you and your radio are down on the
ground. Climbing up on the roof to listen to the radio or watch TV is inconvenient, to say the
least. A transmission line solves the problem of how to move radio frequency signals down to a
receiver or up from a transmitter.
A transmission line is a pair of parallel wires separated by insulation. It works very
much like a speaking tube in a ship or even the string in a tin can telephone. In all these devices,
vibrations are transmitted down a narrow pathway with surprisingly little loss of energy. A
terrific example of a mechanical transmission line can be a farmer’s ditch filled with water.
Provided the water is flowing slowly, when you throw a big rock into the ditch, the wave from the
splash will travel hundreds of feet before it dissipates. A wave in the ditch propagates unchanged
for many minutes and travels great distances. In contrast, if you throw the same rock into an
open pond, the wave spreads out in all directions and quickly vanishes.
A radio transmission line is a distributed, LC resonant circuit. We saw in Chapter 2 that a
simple wire has inductance. Similarly any two wires separated by an insulator comprise a
capacitor, whether we planned on making a capacitor or not. Consequently, when we run two
parallel, insulated wires over any distance, there will be a measurable capacitance between them
and the wires themselves will have a significant amount of inductance. To a radio wave, this
10. Chapter 4, Harris
construction looks like a long, continuous LC circuit. As the capacitance and inductance are
charged and discharged, the oscillation doesn’t stand still, but rather moves down the pair of
wires at nearly the speed of light. As you can see, propagation down a transmission line is
analogous to propagation through free space, but it only propagates in one dimension instead of
three dimensions. The electric field or voltage generates a current and magnetic field, which in
turn generates a new electric field and so on. An example of a simple transmission line consisting
of two parallel wires is the flat wire "300 ohm" twin lead used to feed older TVs.
Coaxial cable
The round, shielded transmission line used to feed modern TVs is a coaxial cable. Instead
of using two separate ordinary wires, the outer conductor of a coaxial cable is a metal cylinder
that completely encloses the center conductor. The inductance of the shield conductor is far less
than that of a simple wire, but it prevents the radio frequency signals on the inner conductor from
leaking out. Even better, it shields new signals from leaking into the cable and interfering with
TV reception.
The ham radio dipole antenna described earlier uses type RG-58 coaxial cable to transport
the radio frequency signals down into the house. You will notice that the outer shield is
connected to ground. This is nearly always the case with coax. You could use cheap TV coax
cable for your ham transmitter, but you will find it difficult to work with. The outer shield of
inexpensive coax is just aluminum foil and is difficult to connect mechanically and electrically. In
contrast, the outer shield of quality coax is braided copper wire that is easy to cut and solder. It
also has considerable mechanical strength.
Transmission line "characteristic impedance"
An abstract characteristic of transmission lines is that, from the point of view of a traveling
radio signal, the line “looks like” a specific load resistance. For example, RG-58 coax appears to
the radio signal to be a 50 ohm resistor. It isn’t, of course, but the voltage and current levels
along the wire suggest that it is. In other words, voltage divided by current at points along the
line will give 50 ohms. Another reason not to use TV cable is that TV cable is usually designed
for 75 ohms, while most ham equipment is designed for 50 ohms. Actually, the difference in
performance would be minimal.
The old flat, brown, TV lead-in wire had a characteristic impedance of 300 ohms. In
general, the finer the wire and farther apart the two conductors of a transmission line, the higher11. Chapter 4, Harris
the characteristic impedance. Sometimes hams use a wide “ladder line” in which bare copper
conductors are separated by an inch or more of air and ceramic separators that maintain the
distance between the wires. Ladder line often has an impedance of 600 ohms. Ladder line is
useful when transmitter power must be transmitted great distances to get to the antenna. Because
a ladder line has little or no insulation in contact with the wires, the small dissipation of energy in
the insulation is reduced to the absolute minimum. More importantly, the high impedance means
that smaller currents will flow in the wire and less energy will be dissipated in the resistance of the
copper. Of course the trade off is that higher voltages are needed to transmit the same power
levels. I know a ham who lives at the bottom of a canyon where radio reception is poor. He uses
a long ladder line hung in the trees to connect his transmitter to the antenna which is located
hundreds of yards up on the mountainside.
Diode detectors
The diode is the detector that converts radio frequency sinewaves into audio frequency
electric waves, ready to be converted into sound. The diode is a “one way electricity valve.” In
plumbing terms, it works like a check valve. The schematic symbol for a diode is an arrowhead
pointed at a barrier at right angles to the wire.
In electronics, the convention for “positive” current flow is from positive to negative.
Unfortunately, the flow of electrons is from negative to positive. So, what is actually “flowing”
from positive to negative is the absence of electrons. Confusing, no? I suspect this convention
was established before electrons were understood. Referring to the symbol for a diode, positive
current is allowed to pass if it flows in the direction of the arrowhead. Positive current will be
blocked by the diode if it attempts to enter the diode from the perpendicular “barrier” side.
Semiconductors usually perform the check-valve function of diodes. A semiconductor is a
crystal of an element like silicon or germanium that has a chemical valence of 4. That is, during
chemical reactions this element can either take up 4 electrons, or give away 4 electrons. When
atoms react with each other to form a molecule, they share these outer electrons to form a stable
outer "shell" of electrons. For most pure atomic elements, this outer stable shell needs 8 electrons
to be stable.
Extreme examples of stable elements are the inert gases; neon, xenon and radon. These
atoms already have 8 electrons in their outer shells. For all practical purposes, these gases are
totally inert and will not react with other elements to form molecules. As we shall see shortly,12. Chapter 4, Harris
semiconductors can also be fashioned by making crystals out of elements with either 4 outer
electrons or mixed elements with valences of 3 and 5, or even 2 and 6. Notice that the valences
of the mixtures still add up to 8 electrons.
N-type semiconductors
To keep it simple, let’s suppose we have a pure crystal made out of silicon semiconductor,
which has a "valence" of 4. The valence is the number of electrons loose enough to become
bound to other atoms in a molecule. If we put some multimeter (ohm meter) probes across pure
silicon, it will act like an insulator. There will be no significant current flow. The valences of
pure silicon atoms in a crystal add up to 8 electrons because each silicon atom is sharing electrons
with 4 other atoms so they all experience stable 8 electron outer shells.
However, if we make a new silicon crystal with just a touch of phosphorus impurity in it,
suddenly the crystal becomes a conductor. Phosphorus has a valence of 5 and is almost the same
as silicon in atomic weight. This means that in chemical reactions phosphorous normally accepts
3 electrons to complete an outer shell of 8 electrons. But when silicon crystal is contaminated
with phosphorus, lone atoms of phosphorus are trapped among a frozen, rigid crystal of silicon.
The phosphorus atom fits in the matrix, but it has an extra electron that is “loose” and free to
move around the crystal. The electron can’t move over to silicon atoms because they are joined
with neighboring silicon atoms so that each silicon atom has a stable outer shell of eight shared
electrons. However, the extra phosphorous electron can move over to other phosphorous atoms
that have already lost their 5th electron. In other words, a silicon crystal with just a touch of
valence 5 impurity acts like metal. It has electrons that are free to migrate through the whole
solid. A semiconductor with extra electrons is called an N-type semiconductor.13. Chapter 4, Harris
P-type semiconductor
P-type semiconductor is a bit abstract. Instead of making a silicon crystal with valence 5
impurity, now suppose we add an impurity such as aluminum, indium or gallium with a valence of
3. The impurity fits into the crystal matrix, but it needs one more electron to reach an equilibrium
of 8 electrons shared with its neighboring silicon atoms. In other words, this semiconductor has
“holes” in the crystal matrix that can be filled by electrons passing through. Now when you place
multimeter probes across a P-semiconductor, it will conduct just like the N-type semiconductor.
However, the conduction mechanism is different. With P-type semiconductor, the negative metal
probe touching the crystal supplies all the free electrons flowing through the crystal. These
electrons are hopping from hole to hole to cross the crystal.
Like phosphorus, aluminum atoms have almost the same atomic weight and size as silicon.
Aluminum atoms fit perfectly in the silicon crystal matrix.
Diodes are P-N junctions
Semiconductor diodes are constructed by placing P- type semiconductor in contact with
N-type semiconductor. In other words, for electrons to flow through the diode, the electrons
must enter the N-type crystal and then move across the junction into the P-type where they
complete the journey by jumping from hole to hole.14. Chapter 4, Harris
“positive to P conducts”
If we measure the resistance across a diode with an ohmmeter, the resistance is low when
we place the positive pole of the meter on the P-type semiconductor and the negative pole on the
N-type. OK. Now let’s reverse the probes of the ohmmeter: We are placing the positive probe
against the N-type semiconductor and the negative probe against the P-type. Electrons flow off
the metal probe and into the P-type semiconductor. No problem so far. On the other side of the
diode the extra electrons from the N-type silicon are being attracted or “sucked” into the positive
metal probe. Therefore the conduction seems to start out all right, but it isn’t long before the
extra electrons in the N-type silicon along the P-N junction are depleted. All that remains in this
region are depleted valence 5 atoms that are now acting like pure silicon. This whole region now
acts like pure silicon and the conduction stops.
So why can’t the electrons that are migrating through the P-semiconductor holes hop
across the P-N barrier and move onto the valence 5 atoms? The reason is the same. The
migrating electrons have filled in all the holes in the P-type and the crystal has also become
pseudo-pure silicon which is an insulator. At the same time, the extra electrons from the valence
5 contaminated semiconductor have all migrated out of the crystal matrix. Both halves of the
diode have been converted into insulators. When thinking about PN diodes, remember, “positive
to P conducts.”
Real diodes
Commercial diodes come in all sizes. The types suitable for detectors in crystal sets are
the little bitty guys on the lower left. The big black diodes on the right are rectifier diodes and
diode "bridge" arrays for power supplies. The long black diode is rated at 6,000 volts. The two
diodes that are built like machine bolts are high speed, high power units that might be used in a
large switching power supply or an industrial RF generator.15. Chapter 4, Harris
Detection of AM radio signals with a diode
In amplitude modulation, (AM) the audio speech signal is impressed onto the radio signal
by varying the AMPLITUDE of the radio signal. An AM transmitter literally increases and
decreases the output power of the transmitter in time with the speech and music being broadcast.
The drawing below shows an unmodulated radio signal of the sort used to send Morse code.
The radio frequency sinewave remains the same amplitude throughout the time that the
transmitter is keyed. Because the sinewave maintains its amplitude during the “dots” and
“dashes,” Morse code signals are known as continuous wave or “CW.”
As the name implies, an amplitude modulated (AM) radio broadcast, (550 KHz to 1.750
MHz ) makes a continuously varying graph of the RF signal that looks like a psychiatrist’s
Rorschach. But of course the outline of the audio signal is actually made up of hundreds of
thousands or millions of RF sinewave cycles.16. Chapter 4, Harris
The diode detector recovers the audio signal by “shaving off” one of the two polarities of
the RF signal. Sinewave currents have both positive and negative polarity. Diodes only allow
conduction in one direction. So, when a radio frequency sinewave current is passed through a
diode, one of those polarities will not pass and will be eliminated. What remains is a series of
narrow, direct current pulses, all with the same polarity.
This detection process, rectification, produces a varying DC signal that may be passed
through a headphone to convert it into sound. If you try to pass radio frequency current through17. Chapter 4, Harris
a headphone or loud speaker, no current will flow and there will be no sound. The coil electro-
magnet in these devices is an inductor which has very high "impedance," AC resistance to current
flow. By rectifying the radio frequency current, there is just low frequency audio frequency
remaining. Many crystal sets put a small capacitor across the headphone to smooth out and
remove the thousands of "bumps." In my crystal sets, there was no difference in loudness with or
without the capacitor.
Physically, a modern detector diode is usually a tiny glass cylinder typically ¼ inch long
with two wires extending from the ends. It's not much to look at. As will be described below, it’s
more fun to make a diode out of sulfide ore, or even out of razor blades or safety pins.
Headphones
After the diode has generated the varying DC current representing the audio signal, a
device is needed to convert the current into sound. The classic way to do this is to use a magnetic
headphone. As we shall describe below, a headphone is an electromagnet that attracts a thin, steel
diaphragm and makes it vibrate in time with the speech and music.
A practical crystal set schematic
The crystal set can be extremely simple. A schematic is shown below:
Crystal Set Parts List:
Big antenna – 50 feet of wire strung up in a tree will be ideal. Or, use the 40 meter dipole
described earlier. For this application, use the entire dipole assembly as if it were a single piece of
wire. Connect the center conductor and the braided outer shield of the coaxial cable together and
fasten the resulting “wire” to the “big antenna” location above.
Good ground – An ideal ground can be obtained by clamping the wire to a household copper
water pipe. Alternately, you may use a second length of wire strung up in another tree. Ideally,
the second wire should be far away from the first wire. I happen to have a 30 meter ham band
dipole in my backyard. I used my 40 meter dipole in my front yard as the “antenna” and the 30
meter as a “ground.” Or, as it would be called in this application, the 30 meter dipole became a
”counterpoise.”
Inductor - Wind about 20 turns of bare copper wire around a large diameter cardboard tube.
Cardboard Quaker Oats boxes are the classic coil form for this purpose. The cardboard cylinder
from a toilet paper roll will also work, but more turns are needed to produce the same inductance.
I used to believe that large coils should work better because the coil is acting as a magnetic
antenna, as well as a tuned LC circuit. In other words, a large diameter coil ought to be able to
snag more magnetic field component from the radio wave. As you'll see later in the chapter, I was18. Chapter 4, Harris
unable to demonstrate the directionality of large coils at AM frequencies. Live and learn. To
tune the crystal set, you need to rig up a slider or shorting clip that allows you to short out some
of the coil.
Capacitor - Where’s the variable capacitor? A capacitor consists of two pieces of metal
separated by an insulator. If you wind a big coil of wire around a cardboard tube, then there is
capacitance between one loop of wire and all the neighboring loops. “But, hold on! That can’t
be! They’re shorted together!” you say. Yes, you’re right. But if you look at an LC circuit as a
whole, the inductor is a kind of “short circuit” across the whole capacitor and we know that
works OK. The hard part about physics is that you often need to think abstractly. Many
phenomena seem fuzzy and inconsistent. We are forced to “get a feel” for what works and what
doesn’t. The coil of wire is said to have inter-winding capacitance that acts the same as if it
were a separate capacitor across the whole thing, honest.
Crystal diode - It’s great fun to build your own diode as described below. However, to get
started, you may want to use an ordinary, small silicon diode such as a type 1N4148 or a 1N914.
They are available at Radio Shack and any other electronics semiconductor vendor.
Headphones. - You may construct a working headphone from ordinary materials as described
below. This will be entertaining and educational, but eventually you will need to buy a good pair.
You may buy either old-fashioned high impedance (2000 ohms) headphones or modern low
impedance (8 ohms) headphones. The modern ones are extremely efficient, comfortable to wear
and have hi-fi sound. They are also built from coils and magnets, their design is obviously much
more sophisticated. The high impedance magnetic headphones are historic and little more can be
said in their favor.
******************************************************************************
Homebuilt diode detectors
My experimentation with crystal sets as an adult began one day when I was hiking near
Jamestown, Colorado. I was scrambling up a yellow-colored abandoned mine dump. Mine
tailings up there are mostly yellow, sulfated, powdered rock that consists of broken-down granite
or gneiss. Suddenly right in front of my face were chunks of the shiny, black sulfide ore that was
the reason for the mine. Without an assay, I don’t know exactly what’s in this ore, but it’s a safe
bet that it’s a mixture of sulfides of silver, lead, maybe zinc, a dash of arsenic, tin and copper.
There might even be a trace of gold telluride in those crystals. Galena, lead sulfide, is the classic
material used in old-time crystal sets to make detector diodes. I thought, “Gee! I wonder if I can
make a crystal set out of this ore?”
It seems to me I once saw a war movie in which a POW in a Nazi Stalag made a radio out
of barbed wire, a razor blade and silver paper from a chewing gum wrapper. Well, that’s
Hollywood, but maybe a receiver can be built without using parts specifically manufactured for
radios. I happened to have a toy crystal set radio dating from about 1940 in my attic, so I hauled
it down and checked it out. The “diode” consists of a tiny chunk of gray galena sticking out of a
little puddle of solidified solder. The positive pole of the diode is a metal “cat whisker,” a piece of
thin copper wire poked against the crystal.
My crystal set project was done in three sessions and is written as separate articles.19. Chapter 4, Harris
Before you build the set pictured below, I suggest you read about the crystal set as described in
the second article following this one. The second crystal set encountered some abstract mysteries,
so although it worked better, it is harder to understand.
The above picture shows my crystal set with the three basic homemade components. The
black "phone plug" connects the homemade earphone to the circuit. I used this standard
connector so I could plug in a commercial headphone for comparison with the homemade
headphone.
An LC resonant circuit is vital to select the AM radio band (or other band). An actual
tuning capacitor isn’t really needed at AM radio frequencies. A big coil, at very least two inches
in diameter with 40 to 60 turns wound on a cardboard tube, has enough inter-winding capacitance
to resonate in the AM band. From a physics point of view, the circuit in the “practical” schematic
is functionally the same as the circuit that contains the variable capacitor across the coil.
Without a variable capacitor, you will have no way to tune in particular stations. A tap on
the winding can be added for peaking a station. A “tap” is just a way to short out part of the
inductor. Using this method you can crudely, very crudely, select the loudest stations at the top
or bottom of the AM broadcast band. Tuning a crystal set is (usually) sloppy. However, the
article at the end of this section demonstrates a way to accomplish sharp tuning.
The crystal diode rectifies the radio frequency voltage ringing on the LC circuit and the
headphones turn it into sound. Some crystal sets also have an audio signal filter or “integrator”
capacitor. This capacitor, about 0.01 microfarad, is placed across the headphones. In theory, it
converts the millions of little radio frequency halves into a smooth, low frequency audio
waveform. However, in my crystal set it didn’t do anything I could notice, so I took it out.
Leaving out parts is a good way to find out what they do.
Try leaving out the LC circuit and just connect the diode and headphone to the antenna
and ground. At my house all I could hear was faint static that sounded like power line noise.20. Chapter 4, Harris
That implies that power lines generate the biggest AM signals over the entire radio spectrum. In
any case, without the LC circuit, I heard no radio stations.
The Jamestown crystal diode
To make my crystal detector out of sulfide ore, I melted a puddle of solder about 3/8 inch
wide on a small piece of blank printed circuit board. Then I used tweezers to press a bit of ore
into the puddle so that, when it hardened, half of the crystal was exposed. Next I soldered a thin
semicircle of copper wire onto an isolated copper pad on the board for a "cat whisker." The
copper wasn’t springy enough to poke into the crystal with enough force for reliable performance.
It worked well, but with a little vibration, it quickly died.
A safety pin pushes the copper whisker against the galena.
In my next diode I made a copper ring cut from the end of 3/8 inch copper tubing. This
served as a deep “tub” of molten solder into which I could push the galena. Bob, NØRN, told me
that when he was a kid, he used safety pins as cat’s whiskers. Sure enough, the spring-loaded
safety pin produced plenty of force and solved the mechanical problem. Don’t forget to cut the
notch or gap in the circuit board between the galena side and safety pin side – they are two
separate nodes in the circuit!
Carbon steel is a semiconductor21. Chapter 4, Harris
When I first soldered my diode with the safety pin cat whisker into a crystal set, it was
stone silent - nothing. No matter how I moved the sharp steel pin around on the galena, the
headphones were dead. The pin happened to strike the solder at the edge of the galena and the
crystal set came to life! I heard music from KBCU, our loudest local AM station. At first I was
mystified. The steel pin rectified well against either solder or copper. The signal was perhaps
only 2/3 as loud has it had been with the copper-to-galena diode, but it was much easier to adjust.
It turns out that steel is a carbon-iron semiconductor compound called cementite. The
surface of hardened steel is a crystal, perhaps not radically different from the galena crystal.
Carbon has a valence of four, just like silicon or germanium. Iron exhibits a valence of +2 or +3,
but apparently that's close enough for this crude semiconductor. So, if you want to build a crystal
set for your kids, you don’t have to mine galena. Just use a safety pin pressing against copper
or solder.
Another surprise for me was that copper-to-copper, solder-to-solder, or solder-to-copper
junctions also rectify and produce weak signals. The contact between the two metal surfaces
must be extremely light - just barely touching. This phenomenon is poor for making crystal sets,
but it’s a warning about bad contacts in electronic equipment. Cold solder joints and loose
screws could fill your circuit with accidental diodes.
Copper cat whiskers work best
As shown in the diode construction diagram, I used acid core solder to attach a piece of
copper wire onto the end of the pin. Rosin core solder sticks poorly to steel. Now the contact
point of my diode is between semiconductor galena and copper, rather than semiconductor steel-
to-semiconductor sulfide ore. I connected my crystal set to the center conductor of my 40-meter
dipole coax and my station ground. I scratched the copper whisker around on the sulfide crystal
and suddenly I was again hearing our local station. Using commercial 8-ohm headphones, it was
almost painfully loud. Too bad KBCU is mostly rap music.
Where is the P-N junction in these crude diodes?
If you are a thoughtful person, you must be asking, “Where is the P-N junction with the
impurities imbedded in the pure semiconductor and all that?” It turns out that you can make
crude diodes by throwing together rather inferior materials. For example, pure galena crystal
consists of lead and sulfur that have valences of 2 and 6, that sort of average to 4. But there are
also all those other atoms in typical galena ore. These impurities, like silver or copper, have
valences like plus 1, while other transition metals like tin have valences of 2 or 4. Let’s just
assume that because of sulfide (valence 6) and arsenic (valence 5) salts, my ore is a N-type
semiconductor. So where is the P-type semiconductor? It turns out that if you press a pure metal
against N-type semiconductor, metal ions will migrate a few microns into the N-type crystal. This
makes a tiny P-type region surrounding the contact point of the “cat whisker.”
As you might expect, the disadvantage of such crude diodes is that P-N junction is quite
fragile. That is, the check valve function only works with very low voltages and extremely small
currents. The P-N junction is easily destroyed if you put large reverse voltage across it or try to
pass large currents through it.
I tried measuring the volt/ ampere characteristic of a steel/ copper diode. It was no
surprise that it looked like a short circuit on an ohm-meter. I studied it carefully with a high22. Chapter 4, Harris
impedance multimeter and 10 megohm series resistors, but it still looked like a short circuit. I
believe I just learned that the world of RF detection is quite subtle. At least the legend about
POWs in WWII making radios out of barbed wire and razor blades is starting to make sense. The
razor blade must have been the semiconductor.
Carborundum diodes
Carborundum, silicon-carbide, is an rock-like abrasive made by cooking silicon sand and
carbon in an extremely hot oven. It's that black stuff on emery paper. Silicon is a semiconductor
and a crystalline mixture of the two is also a semiconductor with unique properties. Ashish,
N6ASD, gave me a sample to try out. It is a jet black, shiny crystalline material. The flat crystal
facets have a blue-green and red reflective sheen, like oil on water.
The most primitive LEDs
A curious property of silicon-carbide is that it can be used to make a crude, amber-colored
LED. You apply a fairly high voltage to a piece of carborundum and, if you're lucky, the crystal
around the contact point will glow weakly. Ashish demonstrated this using my 45 volt DC
supply. After he left, I managed to make it glow amber too. Since then, I haven't been able to
find any "magic spots" that glow. I did manage to heat up small crystals until they glowed red
hot, like a glowing wooden match stick when the flame is first extinguished. However, the
current was so high and the crystal so hot, it might as well have been a carbonized match stick. It
was definitely not LED action. Perhaps this is why LEDs had to wait for the modern era.
In recent years, carborundum has been used to make high power Schottky diodes and blue
LEDs. Although it sometimes occurs in nature, it is nearly always manufactured. I have read that
the detector in the Titanic receiver used carborundum cat-whisker diodes. I made some diodes
out of small pieces and yes, they worked well, but were much harder to adjust. The cat-whisker
usually must be applied to the edge of a crystal rather than the shiny flat surfaces. Ashish told me
that he read that biasing carborundum detectors with DC can produce a little more sensitivity. I
did observe a slight improvement when I added a few milliamperes of DC bias. In summary, I
prefer my crude sulfide ore diodes, but I'm no expert.
Commercial diodes
A perfect diode would have zero resistance in the forward direction and infinite resistance
in the reverse direction. As you can see in the graph below, no real diode is perfect and all diode
types have different forward voltage characteristics, depending on the semiconductors used to23. Chapter 4, Harris
make them. The reverse characteristics of commercial diodes are all excellent and similar, but
each type of semiconductor diode needs a certain level of forward voltage before it will begin to
conduct. Commercial diodes handle hundreds of milliamperes or even hundreds of amperes. In
the graph below relative current is plotted on the vertical axis. All three diodes were identical
(purple) on the negative horizontal axis.
Commercial diode characteristics
Volt – ampere characteristics of homemade diodes
Once I had my cat whisker adjusted, the Jamestown diode was just as loud as a modern,
hot-carrier Schottky diode. In theory, a Schottky should be comparable to a point-contact
(galena crystal set) diode. Schottky diodes are commonly used as detectors in relatively modern
equipment. If these diodes perform the same, how do their volt/ ampere characteristics compare?
As you'll see below, homemade diodes can barely handle microamperes of current before they fail.24. Chapter 4, Harris
Volt / Ampere curves for the Jamestown diode and a commercial Schottky diode
At big voltages and currents, (milliamperes), the Schottky diode acted as you would
expect: It passed big currents (milliamps) above 0.2 volts forward voltage and leaked only 100
nanoamperes with reverse voltage. Notice that if the Schottky were “perfect," the red line would
be directly on top of the vertical axis upward, and directly on top of the horizontal axis to the left.
However at very low currents, microamperes, the commercial Schottky was pretty nearly perfect
with a transition right at zero volts.
In contrast, with big currents the Jamestown diode behaved like a resistor in both
directions. At first it wasn’t obvious to me that it could rectify anything. The curves above show
the behavior of both diodes at tiny current levels, microamperes, using a 1 megohm (one million
ohms) load. For tiny currents, the Schottky and Jamestown diodes were both strongly non-linear
at the zero current, zero voltage point. The surprise for me was that, for reverse voltages, the
Jamestown diode broke down abruptly at minus one volt. It’s no wonder it conducted so well in
both directions with a “low” resistance 10K ohm load. This abrupt, reverse breakdown is called
avalanche breakdown. When it occurs with big currents, it usually produces so much heat that
it destroys the diode. As we’ll see in Chapter 8, some diodes called Zener diodes are designed to
breakdown at specific voltages without being destroyed.
A homemade headphone
Building my own headphone was the most difficult part of my crystal set. A headphone
uses a high impedance coil of wire to make a magnetic field proportional to the audio signal. The
changing field pushes and pulls against a thin steel diaphragm to produce sound vibrations. Even
if you decide to build one of these headphones, I strongly suggest you buy a good pair of
headphones so you will have them for your ham rig. Also, with commercial headphones the
speech and music will be perfectly clear and loud in the crystal set.25. Chapter 4, Harris
The Caribou headphone
A cross section of my homemade headphone is diagramed above. Its construction is
basically the same as old-fashioned high impedance headphones. Yes, the sound is tinny. What
did you expect from a headphone diaphragm made from a tin can lid? The coil is hundreds of
turns of #36 wire wound on a paper coil form. Inside the coil is a cylindrical magnet I took out of
an old loudspeaker. A piece of steel strap conducts the magnetic flux around to the edges of the
lid. The magnetic force holds the lid on. By completing the magnetic circuit loop, the magnetic
force is concentrated in the gap between the tin-plated steel and the magnet.
Crystal set showing homemade headphone. The tin can lid diaphragm has been removed.
I started out using a small magnet from an old loudspeaker, but that felt like cheating.
Would Heinrich Hertz have been able to use a loudspeaker magnet? Anyway, it seemed to me
that the magnet wasn’t essential. Why couldn’t the coil just magnetize ordinary iron? I tried
substituting a big steel nut of the same size. Sure enough, it worked, but the sound was too faint
to be intelligible in the crystal set. However, when I plugged the homemade headphone with the
steel nut into my shortwave radio, it was surprisingly loud. Not Hi-fi, mind you, but loud.
Clearly a sensitive headphone needs a magnet to overcome the hysteresis.
Hysteresis
What’s "hysteresis," you ask? A mechanical analogy is the meshing of two loose gears,26. Chapter 4, Harris
perhaps in an automobile transmission. If the gear teeth are not in close contact, then when one
gear begins to turn, there will be a slight delay before the teeth engage the second gear. When the
direction of the first gear reverses, there is another delay while the teeth travel across the gap and
engage the second set of teeth. On the other hand, if the first gear never reverses direction, the
teeth remain engaged and there are no delays.
Iron core inductors also have this delay problem. Whenever iron is magnetized with a DC
coil, the tiny “magnetic domains” in the iron line up to make a big magnetic field. But when the
DC current is shut off, some of the magnetic domains remain aligned and leave a residual field.
To magnetize the iron in the opposite direction, a current of the opposite polarity must first
overcome the residual field. This means that hysteresis interferes with the sensitivity to weak
signals. Since crystal sets are powered by the radio waves themselves, sensitivity is vital. A
magnet is needed to overcome the hysteresis and “bias” the magnetic field so that it always
operates in one direction. I could magnetize iron with a DC powered coil, but then to be a purist,
I would need to build a homemade battery. And I would need to smelt and extrude my own
copper wires. Unfortunately, or perhaps fortunately, there are limits to what we can do in our
basements.
I had a sudden inspiration. I dug around in my rock collection and found a piece of
magnetite ore from a mine dump at Caribou, Colorado. Magnetite is a specific iron oxide, Fe3O4,,
that retains a magnetic field. It is grey-black in color and surprisingly heavy. I machined the
magnetite with my bench grinder into a small cylindrical magnet. Unfortunately, the grinding and
heat ruined the magnetism. However, fixing it to a big, heavy permanent magnet, I was able to
put my magnetite in a strong magnetic field. Then I banged on it firmly against my anvil. Believe
it or not, that abuse restored the magnetic field. Behold! - The completed Boulder County rock
and toilet roll radio!
How does it perform? Well, frankly the homemade headphone is pathetic and needs lots
of R & D. The sound is plenty loud when plugged into a real radio, but installed in the crystal set,
I can just barely hear the rap music. Perhaps if I had a thinner steel diaphragm, a headphone for
each ear, optimum impedance matching, better craftsmanship and other refinements, it might
approach a commercial headphone. In other words, for serious listening, buy a decent
headphone! And, after you're done playing with homemade crystal diodes, I suggest you buy
some silicon diodes. Ordinary type 1N914 or 1N4148 diodes work great in this radio. They do
NOT work better than the diode made from sulfide ore, but they are smaller, more rugged and
don’t need to be carefully adjusted.
This crystal set article was first written in 2002. I revisited crystal sets in 2006 and 2017
and discovered some surprises.
******************************************************************************
CRYSTAL SETS REVISITED
Or, electronics makes us humble
A friend of mine, Jack Ciaccia, WMØG, suggested to his grandson, Rutger Koch, that a
broadcast band crystal set like the one described above would be a good science project. Rutger
did a terrific job of duplicating the set and the homemade diode. The only obvious difference I27. Chapter 4, Harris
could see was that he had used the 5-inch cardboard oatmeal box coil form that I had
recommended. But when Rutger put it together, it was totally inert - no sound.
Rutger consulted his granddad who also couldn't find anything wrong. Jack brought the
crystal set over to my house and we two old hams scratched our heads and still couldn't find the
problem. After five minutes of swapping parts, I dug out my prototype crystal set from the closet
and there was the answer: If you go back to the drawing of the homemade diode made out of the
piece of PC board and a safety pin, you will see that there is a notch carved in the PC board
between the anode and cathode. Rutger had forgotten to cut the copper sheet between the anode
and cathode so they were shorted together. We cut the copper sheet and it worked perfectly.
Our eyes only see what they expect to see! Don't take the obvious stuff for granted. By the way,
Rutger's science teacher refused to believe rocks could serve as diodes until he put on the
headphones. This ancient technology is all new information for whippersnappers.
Does a bigger diameter coil work better?
When I first built my AM broadcast band crystal set described earlier, a toilet paper roll
was the largest diameter cardboard form I could find around the house. Although I hadn't noticed
any obvious superiority in Rutger's crystal set over mine, I decided to wind a big coil like his and
see if my magnetic antenna theory had any validity. That is, the big coil might pick up the
magnetic wave components and produce a bigger signal. Perhaps it would be directional like an
AM band ferrite core antenna, the kind of antenna in your little battery powered radio. Since my
crystal set had previously been converted to 10 meters (see the next article, below), I just took off
the loop antenna and left the 140 pF variable capacitor on the set. Then I installed the oatmeal
box coil with about 25 turns on it. Since I already knew that ordinary silicon diodes have the
same sensitivity as the rock diodes, I used a 1N4148 diode and my large, low impedance
headphone.28. Chapter 4, Harris
Oatmeal box crystal set - operating without a ground or counterpoise
I hooked up the 40-meter dipole antenna, "A" above, and used the 30-meter dipole as a
counterpoise connected to the ground post, "G" above, just as I had before. This was the same
way Jack and I had hooked up his grandson's set. The silence was deafening. Good grief! Now
what?
After some flailing about, I accidentally disconnected the counterpoise wire as shown
above, just leaving the antenna wire connected. Voila! It not only worked well, the tuning was
precise enough to tune in my local NPR station at 1490 KHz with excellent selectivity and signal
strength. Specifically, when tuned to 1490, there was no rap music from 1190 KHz! Tuning was
not only loud and clear, I could only hear the one station and not 2 or 3 at once. Precise tuning
with a crystal set! How is this possible? When I padded the relatively small 140 pF variable
capacitor with 100 pF fixed capacitors, I was able to tune down the AM broadcast band and
uniquely select other stations. The farther down the band I went, the less precise the tuning. I
have no good explanation for that. If I were starting over, I would use a larger variable capacitor,
e.g., 365 pF.
How does this work?
The selectively tuned crystal set seems to be an example of a circuit that doesn't seem to
have a complete circuit loop. Without a ground or a counterpoise, where does the current go? I
assume that there is some form of capacitive coupling that completes the loop, but I admit this
coupling isn't obvious. As you can see, RF sometimes makes current loops that are hard to
identify.
Using the backyard antenna as a counterpoise didn't work with the big coil, but I found
that clipping the ground terminal, "G" to my station ground did improve signal strength.
Unfortunately, it ruined the precision of the tuning. Hooking it to the station ground and the
counterpoise antenna together still worked, but was weaker and produced poor tuning. Using the
backyard 30-meter dipole as the sole antenna, worked just as well as the 40-meter antenna.
I probed with my oscilloscope at various places on the circuit to get some answers. As
soon as I clipped on the oscilloscope probe ground wire, it behaved just like the grounded hook
up, just as you would expect. The signal strengths across the coil were on the order of 0.40 volts
peak. The power delivered to the 8-ohm headphones was about 20 milliwatts peak. The most
surprising observation was that, when I just left the scope probe and its ground wire lying six
inches away from the crystal set, I could see the modulation on the oscilloscope screen appear and
disappear as I selected specific stations. The voltage signal was only slightly less than when it was
directly connected. Apparently there is a large electric RF field surrounding the crystal set.
When the RF current passes through the LC circuit and headphone to ground, the current
passing through the diode and headphone is relatively large. This makes lots of sound, but all the
local station signals present pass through at once. When the RF current has nowhere obvious to
go, the RF voltage can only leave by weak capacitive coupling to the surroundings. With no
ground the only source of current for the headphone is the oscillating RF voltage ringing on the
LC circuit. This small RF voltage provides the highest voltage only when the LC circuit is
precisely tuned to a station. Otherwise, you only hear silence.
"Q" is the quality of a capacitor or inductorYou can also read
